This invention relates to a gas metering system for an internal combustion engine. More particular this invention relates to industrial engines for combined-heat-power applications, for powering generating sets and for heavy duty vehicles.
It is generally known to operate industrial engines with gas of various composition. In view of environmental regulations and in view of engine performance, a system is required which is flexible and compensates for different operating conditions, more particularly, for different loads and different compositions of supplied to the related engine.
To that end sophisticated closed loop control systems are known. However, the components of such closed loop systems increase the costs thereof as well as reliability.
A very simple and well known system to feed an engine with gas is to meter gas through a pressure regulator and a venturi provided in the inlet of the internal combustion engine. According to Bernoulli""s law, a constant air-fuel ratio can be guaranteed through the use a pressure regulator functioning at constant (near zero) pressure and equal to the inlet pressure of the venturi. To provide a first adjustment, a main adjusting screw is provided between the pressure regulator and the venturi. In more sophisticated systems, electronic adjustment of the adjustment screw during operation is made possible to optimize the air fuel ratio for all operating conditions.
The invention aims to provide a relatively simple gas metering system for an internal combustion engine which can meet emission requirements at all operating conditions on the one hand and provides easy starting on the other hand.
It is a further aim of the invention to provide a gas metering system, which can be simply adapted to different types of engines. Different types of engines means a different cranking volume and/or different member of cylinders.
It is a further aim of subject invention to provide a gas metering system making operation with various gases and/or gas compositions possible, even with a limited number of hardware executions.
According to one aspect of the invention, a gas metering system is provided for an internal combustion engine comprising a gas pressure regulator having an inlet and gas outlet and being arranged to provide gas at said outlet at a substantially constant pressure, an outlet line being connected to said outlet and comprising an adjustable valve, a venturi device having a main passage for inlet air for said internal combustion engine and at least one auxiliary passage opening in the throat section of said venturi and connected to said outlet line, control means for controlling said adjustable valve, having inputs for at least engine rpm and engine load, a memory/processing unit for converting said inputs into an output for said adjustable valve, and adjusting means for adjusting said control to the related internal combustion engine, said adjusting means comprising setting means for introducing values corresponding to the surface area of said at least one auxiliary passage.
According to the invention, a relatively low number of input/output ports and memory addresses are required so that a relatively cheap processor can be used keeping the costs for the control means as low and the reliability as high as possible. Preferably, the control means comprises a set of parameters for the reference engine to be encountered. This set comprises the cylinder displacement of the engine, gas composition, etc. It has been found that the adjusted values of the total surface area of the auxiliary passage of a subject engine in combination with this set of parameters generate about the same A/F ratio as a reference engine.
It is presumed that for example the venturi supplier determines the surface area of the auxiliary passage from the engine data and fuel composition as specified by the customer. A smaller engine will have a smaller surface area of both the main passage and the auxiliary passage(s) of the venturi. Usually the surface area of auxiliary passage(s) is labeled at the related venturi, can be deduced from its type number, or from its physical layout. Based on that information, in the field a simple setting can be made in the control according to the invention.
The engine load input can be deduced in several ways known from the prior art. According to a further aspect of the invention the metering unit comprises a commercially available pressure regulator providing a somewhat positive outlet pressure in order to improve starting at cranking speed. This is particularly valuable engines having an inlet compressor.
According to a further preferred embodiment, further motorized adjustments means are provided for controlling the air-to-fuel ratio through said valve. As an additional feature, software means are provided to give enrichment at lower engine loads and, more particularly, at a cranking speed of the engine and during a cold start. Also means are provided to make the air/gas mixture leaner at higher intake temperatures.
The motorized main adjusting valve is preferably realized as a slide valve with an opening that is exponentially-shaped giving a nearly constant increase/decrease in surface area at each incremental displacement.
Optionally the metering system can be equipped with a closed loop control feature. The underpressure in the throat of the venturi is a measure of the mixture flow. Only the desired air fuel ratio will generate the desired mixture flow, respectively underpressure, at a given engine power. It is self-evident that the mixture flow will be reduced at rich and increased at lean air fuel ratio conditions. The control means can also have inputs for the measurement of the throat depression and the generated electrical power. The adjustable valve will be controlled as long as the measured underpressure corresponds to the value that belongs to the desired underpressure at that engine power. Alternatively this measured underpressure can be replaced by a calculated underpressure in order to avoid the application of an extra sensor.